1. Field of the Invention
The present invention relates to a PCM digital audio signal playback apparatus for reproducing PCM (pulse code modulation) digital audio signals recorded in a recording medium, e.g. a compact disk.
2. Description of the Related Art
The recording frequencies of a digital audio signal to be recorded onto a recording medium, e.g. a compact disk (CD) or a digital audio tape (DAT), are limited by the sampling theorem to stay lower than a half the sampling frequency for avoiding any folding distortion.
In common, human ears can perceive up to 20 KHz of a single tone (pure sound) and thus, the sampling frequency is 44.1 KHz for CD recording and 48 KHz for DAT recording.
It is now acknowledged through various problems that elimination of a high-frequency component from an audio signal with a filter having a filtering characteristic of acutely blocking frequencies higher than 20 KHz, causes unfavorable results in a reproduced sound and also, a ringing effect in the waveform response to a high-frequency signal such as an impulse will deteriorate the reproductivity of waveforms.
A method for improving the reproductivity of waveforms has been developed in which a short-interval succession of sampling points with time is produced using a Lagrangian or spline function interpolation technique. As the result, CD players or other playback apparatuses which can reproduce a high-frequency signal component higher than 20 KHz, are now commercially available.
Strictly speaking, a high-frequency component reproduced by such a prior art player is rather a folding signal of the fundamental wave than an actual higher-than-20KHz component which is limited by a filter during recording.